<!DOCTYPE html>
<html>

<head>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">
    <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"></script>
    <style>
        div {
            border: 2px solid green;
        }
    </style>
</head>

<body>
    <div class="container">
        <div class="row">
            <div class="col-sm-9"> Level 1: .col-sm-9
                <div class="row">
                    <div class="col-8 col-sm-6"> Level 2: .col-8 .col-sm-6 </div>
                    <div class="col-4 col-sm-6"> Level 2: .col-4 .col-sm-6 </div>
                </div>
            </div>
        </div>
    </div>
</body>

</html>